The present invention relates to improvements in controlling the sensitivity and testing the operation of a metal detector apparatus of the type disclosed in U.S. Pat. Nos. 4,433,528 and 3,972,156.
The above-mentioned patents disclose a metal detector apparatus suitable for use in a forage harvester for protecting the crop processing unit. The crop processing unit includes a crop cutter head which rotates at high speed to cut or chop the crop, and if hard metal objects were to be fed into the crop processing unit the blades of the cutter head could be severely damaged. Even small objects which may pass through the crop processing unit without damaging the unit may harm livestock if the crop is used for feed. To prevent the passage of ferrous metal objects into or through the crop processing unit Bohman U.S. Pat. No. 4,433,528 provides a metal detector for detecting objects and stopping the crop feed mechanism before the objects are fed to the cutterhead.
The Bohman device detects all ferrous metal objects above a given size, the particular size being determined by the sensitivity to which the detector circuitry is designed during manufacture. In some instances it may be desirable for the operator to change the sensitivity of the detector circuitry. For example, the detector circuitry should be set for high sensitivity so as to detect all ferrous metal objects if the crop to be harvested is to be used as a feed for livestock. On the other hand, if the crop is to be used for other purposes, the sensitivity should be set lower so that small metal objects, too small to damage the cutterhead, may pass through the detector without tripping the mechanism which stops the crop feed.
Therefore, one object of the invention is to provide a simple and expeditious way of changing the sensitivity of a metal detector.
It is essential that the metal detector circuitry and the feed stop mechanism operated thereby be correctly adjusted and in proper operating condition, but presently available methods for testing function and sensitivity are, at best, rather crude and imprecise. For example, one presently used method of testing sensitivity involves replacing the metal detector control box with a test box, operating the harvester feed with pieces of baling wire attached to the feed roll so as to obtain a desired output signal level from the test box, and then reinserting the metal detector control box and operating the feed mechanism with the wire still attached to see if the same output signal level is obtained from the metal detector control box. If the same output signal level is obtained then the circuitry of the metal detector control box is over-sensitive and should be replaced. Clearly, this method of checking sensitivity is timeconsuming and is difficult for an operator to perform unless he is familiar with electrical circuitry. Therefore, a further object of the invention is to provide a simple method and apparatus whereby an operator lacking skills in electronics may check the function and sensitivity of a metal detector.